THIS invention relates to a temperature sensing device and a method of producing such devices. In particular the invention relates to a negative temperature coefficient thermistor.
Printing of functional inks has a long tradition in the electronics field. For example, pigment based inks are used to screen-print interconnections and resistors on printed circuit boards. In these applications the thick film inks used consist of a vehicle and pigments of silver and carbon respectively, where the pigment particles can have a dimension in the nanometer range.
Traditionally most functional materials have been printed by conventional printing techniques, such as screen printing. More recent developments are aimed at printing not only the passive components of a circuit, but also active components. For example, printed nanoparticulate silicon components are disclosed in International patent application WO 2004/068536, which discloses the printing of semiconducting layers in active devices like solar cells and transistors. However, a commonly known example for an application of semiconducting thick-film pastes is the fabrication of a temperature dependent resistor, known as a thermistor. Such devices are generally considered to be passive electronic components. Of particular relevance here are thermistors which have a negative temperature coefficient of resistance, commonly known as NTC thermistors, meaning that their electrical resistance decreases approximately exponentially with increasing temperature.
Existing thermistors of this general type are composed of pastes comprised of a powder of a compound semiconductor material and a binder material, such as a glass frit. This paste is either screen printed onto a ceramic substrate or cast to form a green body, after which it is sintered at high temperature to form a massive layer or body of semiconductor material. Invariably, because of distortion during the thermal treatment, further trimming of the material to obtain the correct resistance is required before metallization, in the case of thick-film thermistors.
The fabrication processes used place limitations on the substrate materials that can be used, precluding the use of many lightweight, flexible materials such as paper and polymer film. Traditionally, thick-film inks used for the fabrication of thermistors are composed of heavy metal sulphides and or tellurides, such as lead sulphide, and are not compliant with modern legislation such as the European Restriction on Hazardous Substances (ROHS). Recently introduced alternative materials include compositions of mixtures of rare earth and transition metal oxides, such as manganese oxide. Thermistors based on silicon are usually cut from heavily doped silicon wafers, and have a positive temperature coefficient of resistance.
It is an object of the invention to provide an alternative temperature sensing device and a method of producing such devices.